A conventionally known supercharger (exhaust gas turbine type supercharger) comprises a turbine which is disposed in an exhaust gas passage of an internal combustion engine and is driven by energy of an exhaust gas, and a compressor which is disposed in an intake air passage of the engine and is driven by the driven turbine. Accordingly, an air introduced into the compressor is compressed by the compressor, and thereafter the air is discharged toward combustion chambers. That is, a supercharging is performed.
It is well known that the supercharger can substantially compress an air introduced into the compressor, when a flow rate of the air is within a range from a predetermined surge flow rate to a predetermined choked flow rate. Generally, both of the surge flow rate and the choked flow rate increase as a capacity of the supercharger becomes greater. Accordingly, when only one supercharger having a relatively small capacity is used to perform the supercharging, the flow rate of the air introduced into the compressor reaches the choked flow rate under a high load operating condition of the engine, and therefore the supercharging can not be performed. On the other hand, when only one supercharger having a relatively large capacity is used to perform the supercharging, the flow rate of the air introduced into the compressor becomes smaller than the surge flow rate under a low load operating condition of the engine, and therefore the supercharging can not be performed. It is therefore understood that an operating area (load area) in which an internal combustion engine having a single supercharger can be appropriately supercharged is small compared to a whole operating area of the engine.
In view of the above, one of conventional internal combustion engines comprises: a first supercharger having a small capacity; a second supercharger having a large capacity and being connected in series with the first supercharger; a plurality of bypass passages for adjusting an air flow rate or an exhaust gas flow rate supplied to the first supercharger and the second supercharger, and a plurality of control valves disposed in the bypass passages. In this internal combustion engine, both the first supercharger and the second supercharger are appropriately used depending on the operating condition of the engine. This allows the operating area (load area) in which the engine is appropriately supercharged to be expanded.
In the conventional internal combustion engine described above, for example, a control valve (exhaust gas changeover valve) is disposed in a bypass passage for adjusting an exhaust gas flow rate supplied to the turbine of the first supercharger. This exhaust gas changeover valve is controlled by a control apparatus so as to be closed when the load of the engine is low and so as to be opened when the load of the engine is high. This allows the first supercharger having the small capacity to operate mainly when the engine is operated under the low load condition. In the meantime, this allows the second supercharger having the large capacity to operate mainly when the engine is operated under the high load condition. As a result, the engine is appropriately supercharged in a greater operating area, compared to an area where the engine having a single supercharger can be appropriately supercharged.
The control apparatus which the conventional internal combustion engine comprises determines whether or not the exhaust gas changeover valve operates properly/normally in order to retain a state where the engine is appropriately supercharged as described above. Specifically, the control apparatus stores/memorizes “a maximum value of the supercharging pressure when the exhaust gas changeover valve operates normally”, the maximum value of the supercharging pressure being obtained by experiments performed in advance. Further, the control apparatus is configured in such a manner that it determines that the exhaust gas changeover valve is abnormal/anomalous when “an actual supercharging pressure” becomes larger than “the stored maximum value of the supercharging pressure” (see, for example, Japanese Examined Utility Model No. Hei 3-106133).